Sensors based on Brillouin scattering have the ability to measure stress and strain in a medium. This results from an interaction between photons and one or more types of quasiparticles such as phonons in the medium. When the medium is under a strain, whether mechanical, electrical, or thermal in origin, the optical properties of the medium change, causing a certain amount of incident light to be scattered. This relationship, between temperature/strain and the Brillouin frequency shift, is linear and can be exploited for a number of purposes including, e.g., deformation monitoring and health diagnosis of architectural structures. An optical Brillouin scattering sensor embedded within a structure can replace thousands of closely attached and potentially expensive point sensors.
One problem of conventional time-domain Brillouin sensor techniques is their intrinsic limitation of spatial resolution. The Brillouin gain spectrum (BGS) suffers severe broadening and it becomes difficult to resolve the Brillouin frequency shift (BFS) accurately if the optical pulse is shorter than the damping time of an acoustic wave in the medium. This limits the spatial resolution of conventional Brillouin sensors to around one meter and measurement time can be as long as several minutes. Another issue with conventional methods is that the BFS is sensitive to both strain and temperature. Since BFS is linearly proportional to the changes of both variables, it is theoretically impossible to separate them by measuring only one BFS, which results in ambiguity in measurements.
Two general types of Brillouin scattering sensing techniques have been employed, which include spontaneous Brillouin scattering techniques and stimulated Brillouin scattering techniques. The former is used in Brillouin optical time-domain reflectometry (BOTDR) and inherits the limitations in spatial resolution and measurement time inherent to time-domain analysis. The latter includes several techniques, including Brillouin optical time-domain analysis (BOTDA), Brillouin optical frequency-domain analysis (BOFDA), and Brillouin optical correlation-domain analysis (BOCDA). BOTDA is also in the time domain, and while BOFDA improves the signal-to-noise ratio by using synchronous detection, analysis is still performed in the time domain. BOCDA circumvents the resolution limit based on the synthesis of the optical coherence function, which may achieve a high centimeter-order spatial resolution, but needs access to both ends of a fiber under test, which imposes strict limitations in many damage detection systems. For example, if part of the fiber cracks, the measurement can no longer be performed using BOCDA.
Approaches to separating temperature and strain measurements include using a single-mode fiber to measure both the BFS and Brillouin power level, as Brillouin power is also related to strain and temperature. However, the measuring range and resolution of that method are limited by imprecision in Brillouin power measurements. Another single-mode fiber approach uses both Raman and Brillouin signals to separate temperature and strain. However, noise arises from the Raman intensity measurement, and both direct detection and coherent detection that add additional cost and complexity to the sensor.
One final approach is to use multiple single-mode fibers within a single fiber core. However, a large interference between wavelengths leads to poor spatial resolution, limited sensing accuracy, and short sensing distance. Furthermore, the fibers must be maintained at least 40 μm apart to minimize crosstalk, making such an arrangement quite expensive.